Applications dealing with huge amounts of data suffer significant performance impacts when they are deployed on top of an hybrid platform (i.e the extension of a local infras- tructure with external cloud resources). More precisely, through a set of preliminary experiments we show that mechanisms which enable on demand extensions of current Distributed File Systems (DFSes) are required. These mechanisms should be able to leverage external storage resources while taking into account the performance constraints imposed by the physical network topology used to interconnect the different sites. Our answer to such a challenge is the Group Based File System proposal (GBFS), a glue providing the elasticity capability for storage resources by federating on demand any POSIX file systems. Although our first prototype is under heavy development, we discuss in this paper the GBFS model and few preliminary but promising results.